1. Field of the Invention
This invention generally relates to low-temperature solder processes. More particularly, this invention relates to solder pastes used in electronic assembly.
2. Description of the Related Art
Solder paste is widely used in the electronics industry. Solder paste is a combination of a flux carrier and a metallic solder alloy in a powder form. At room temperature the solder paste is compliant enough so that it can be made to conform to virtually any shape. At the same time, it is "tacky" enough that it tends to adhere to any surface it is placed into contact with. These qualities make solder paste useful for both surface mount soldering and for forming solder balls, sometime called solder bumps, on electronic components such as ball grid array (BGA) packages.
During soldering, the flux carrier reacts with and removes oxides from all surfaces involved in the soldering process including the solder pads, solder bumps (discussed in detail, below) and the surfaces of the individual solder spheres that form the solder alloy powder. Once the solder powder begins to melt then molten solder balls coalesce into a whole liquid body. This process is called "reflow". The reflowed solder contacts and wets the solder pads, and, once cooled, solidifies to form a complete solder joint.
Good "wetting" of the solder pad by the reflowed solder is necessary for the formation of a strong bond. Wetting is strongly dependent on the metallurgical reaction between solder and soldering surface, and on the efficiency of the solder paste flux. Wetting starts whenever the molten solder is in contact with clean, oxide-free surface. Therefore, the temperature that solder spheres making up the solder powder starts to melt and the time that the solder is held above the temperature for the flux reaction are important factors for ensuring good wetting and a strong solder joint.
A mixed solder paste for improved wetting is known and described in U.S. Pat. No. 5,382,300 entitled, "Solder Paste Mixture". This mixed solder paste includes mixing eutectic tin-lead solder with various other metals and alloys in order to improve the wetting and strength of the solder joints.
In general, the surface mount soldering process involves placing the electrical contact of an electronic component or substrate, a small amount of solder paste, and a solder-wettable pad on a printed circuit board (PCB) in close proximity. They are then heated until the solder reflows, forming an electrical connection between the solder-wettable pad and the electrical contact of the electronic component. Once the solder has cooled, it forms both an electrical and a mechanical connection between the electronic component and the PCB. This process has numerous advantages over other methods of interconnection. First, a large number of components can be interconnected simultaneously. Second, the process is highly repeatable and relatively low cost and is easily adapted for mass production.
The surface mount soldering process typically begins by stencil printing solder paste onto the solder-wettable pads of a PCB. Once the solder paste is on the solder-wettable pads, the electronic components to be soldered are aligned and set into place on the PCB with the electrical contacts of the electronic components in contact with the solder paste. The solder paste holds the electronic components in place during the heating and reflow process.
Solder bumps may be formed on the solder-wettable pads of an electronic component or a PCB using a method termed contained paste deposition (CPD) described in U.S. Pat. No. 5,539,153 assigned to the assignee of this invention. CPD provides for effective "micro-stenciling" of substances. Using a CPD process, a mask is used to micro stencil solder paste onto the solder-wettable pads. The solder paste is then reflowed and forms into solder spheres, each wetted to a solder-wettable pad. The solder sphere is cooled, forming the solder bump. The mask may then be removed.
While many types of solder paste can be used to perform surface mount soldering and to form solder bumps, the conventional solder paste used contains an eutectic tin-lead solder alloy powder containing 63 percent (%) tin, by weight, and 37% lead, by weight, (63Sn--37Pb). 63Sn--37Pb solder alloy has a melting temperature of 183 degrees Celsius (.degree. C.). Typically, the soldering reflow process temperature peaks 20.degree. C. to 30.degree. C. above the melting temperature of the solder alloy (peak reflow temperature). This ensures the solder on the whole PCB melts completely, flows properly, and wets solder-wettable surfaces adequately, thus assuring quality solder joints. For the 63Sn--37Pb solder alloy, the peak reflow temperature typically is approximately 210.degree. C.-220.degree. C. Such high temperatures can induce considerable strain in a multilayer PCB that the electronic components are usually soldered to. High temperatures can also damage temperature-sensitive electronic components as they are being soldered. Consequently, more expensive components or extra assembly processes are often required in order to avoid damaging the PCB and the electronic components.
In addition, the melting temperature of the 63Sn--37Pb solder alloy may make its use undesirable in some step soldering processes. Step soldering processes are processes in which, in a subsequent operation, electronic components are soldered to a PCB that already has some electronic components soldered to it. Step soldering processes might be used, for example, to solder components on a second side of a PCB that already has components soldered to the first side. For purposes of this description, the electronic components soldered to the PCB prior to the subsequent operation will be called "original components" and the solder alloy holding the original components to the PCB will be called "first solder."
In step soldering processes, the integrity of the solder connections between the original components and the PCB may be compromised if the first solder melts during the subsequent soldering operation. The likelihood that melting the first solder will compromise the integrity of the solder connections is increased both the original components are relatively heavy, and when components are soldered to both sides of the PCB. This increase in the likelihood of compromised solder connections is because the tackiness of solder paste and the surface tension of molten solder is often enough to hold relatively light components in place during a soldering reflow operation, even when the relatively light components are inverted. The tackiness of the solder paste and the surface tension of the molten solder, however, may be overcome by gravity with relatively heavy inverted components. An example of a relatively heavy electronic component is a ball-grid-array package. If both the first solder and the solder used in the subsequent soldering operation are 63Sn--37Pb solder alloy, the first solder will almost always melt during the subsequent soldering operation.
To avoid melting the first solder, the reflow temperature of the solder paste used in the subsequent soldering operation should be at least 25.degree. C. lower than the melting temperature of the first solder. The 25.degree. C. difference between the melting temperature of the first solder and the reflow temperature of the solder paste in the subsequent soldering operation allows two conditions to be met. First, the subsequent soldering operation can occur at 20.degree. C. above the melting point of the solder alloy used in the subsequent soldering operation to ensure quality solder joints. Second, the subsequent soldering operation can occur at a safe 5.degree. C. below the melting point of the first solder. If the first solder is the common 63Sn--37Pb solder alloy with a melting temperature of 183.degree. C., then the solder paste used in the subsequent soldering operation should have a reflow temperature below 158.degree. C.
Some low-temperature reflow solder pastes containing low-temperature melting solder alloys are known in the art. For example, eutectic bismuth-tin (58Bi--42Sn) solder alloy melts at 138.degree. C., a temperature suitable for low-temperature soldering processes including step soldering. 58Bi--42Sn solder alloy does not work well, however, with electronic component leads or with PCBs having tin-lead (Sn--Pb) alloy cladding. Such electronic component leads and PCBs are commonly used in the electronics industry.
When molten 58Bi--42Sn solder alloy wets the Sn--Pb alloy cladding, the lead (Pb) atoms in the cladding diffuse into the molten 58Bi--42Sn solder. The Pb atoms induce a new bismuth-lead-tin ternary eutectic structure (52Bi--30Pb--18Sn) in the solder alloy that melts at approximately 96.degree. C. (.about.96.degree. C. structure). In general, a ternary eutectic structure is a combination of the elements the form the alloy in a ratio that melts at a temperature below the other possible combinations of the elements. In some cases, the formation of ternary eutectic structures can cause portions of the solder alloy that begins to melt before the overall solder alloy melts resulting in a "plastic" phase that is neither completely solid nor completely melted. Ternary eutectic structures can be detected using differential scanning calorimetry. In bismuth-lead-tin (Bi--Pb--Sn) solder alloys, it has been found that the .about.96.degree. C. structures accelerate grain growth as the solder goes through temperature cycles that include temperatures higher than the melting temperature of the .about.96.degree. C. structures. The accelerated grain growth within the Bi--Sn--Pb solder contributes to premature fatigue failure of the solder connection.
Other Bi--Sn--Pb solder alloys are known that are not susceptible to the formation of .about.96.degree. C. structures when used with Sn--Pb clad component leads or PCBs. For example, 43Sn--43Pb--14Bi solder alloy, with a melting point of 172.degree. C., is known. The melting point of the 43Sn--43Pb--14Bi solder alloy is not low enough, however, to be used as a subsequent solder if the original solder is the common 63Sn---37Pb solder alloy.
Solder alloys that include indium (In), such as 40In--40Sn--20Pb and 52In--48Sn, are also known in the art. The indium solder alloys are not subject to the formation of .about.96.degree. C. structures like the Bi--Sn--Pb solder alloys, are compatible with Sn--Pb clad components leads or PCBs, and have a melting point below 158.degree. C. Indium, however, is relatively rare and expensive when compared to tin, lead, and bismuth. The use of solder alloys containing indium can therefore be prohibitively expensive.
Extremely low-temperature melting solders are also known in the art for soldering superconducting devices. These solders include the near-eutectic 56.2Bi--27Pb--16Sn and 52.5Bi--32Pb--15.5Sn alloys that both melt at approximately 100.degree. C. These solders include the .about.96.degree. C. structures and are often not strong enough for many of the conventional electronic components that operate between 25.degree. C. and 75.degree. C.
In addition, when the low-temperature melting solder alloys discussed above are used in a solder paste, any solder joints, solder interconnects, solder bumps, or other solder structures formed by reflowing the solder paste will have a melting temperature at or near the reflow temperature of the solder paste. This raises several concerns. First, if any additional step-soldering processing takes place, the low-temperature melting solder structures are likely to reflow during the step-soldering process unless an even lower-melting solder alloy is used. As discussed above, reflowing previously created solder structures during subsequent step-soldering may compromise the integrity of the connections between the original electronic components and the PCB.
The second concern is that the low-temperature melting solder structures may weaken when exposed to temperatures approaching the reflow temperature of the solder paste used to form them.
Accordingly, it is apparent that there is a need for a low-cost solder paste that reflows below 158.degree. C. and that reflows to form a Bi--Sn--Pb solder alloy that does not include .about.96.degree. C. structures, is not susceptible to .about.96.degree. C. structure formation when used with Sn--Pb clad electronic component leads or PCBs, and has a melting temperature at least 10.degree. C. above the reflow temperature of the solder paste.